1. Field of the Invention
This invention relates generally to the assembly and packaging of integrated circuits for plastic encapsulation. Specifically, this invention is a method for fabricating and mounting an unencapsulated integrated circuit die to a mounting structure such as a metal die paddle that reduces the propensity of the integrated circuit die to crack or fracture when subject to thermal stress.
2. Description of the Prior Art
The current state of the art for plastic packaged integrated circuits describes an integrated circuit die coupled to a mounting structure by epoxy, glue or other types of adhesive material. The integrated circuit die and mounting structure are subsequently encapsulated in plastic material, typically through a transfer molding process.
The problem, noticed in the Joint Electron Device Engineering Council (JEDEC) semiconductor package identified as Thin Shrink Small Outline Package (TSSOP), but applicable to virtually all types of plastic encapsulation of integrated circuits, is cracking or fracturing of the integrated circuit die when subject to thermal stress. Thermal stress can, and often does occur, at temperatures above 25 deg. C. Under test and/or operating conditions, the thermal coefficient of expansion of the mounting structure and the die are very similar, but not identical. Thus, in the absence of plastic encapsulation the mounting structure and the die will illustrate similar expansion and contraction characteristics.
However, once the device is encapsulated in plastic, the thermal coefficient of expansion of the mounting structure is greater than that of the plastic encapsulant. The result of the plastic encapsulation is the presence of competing and opposite forces on the integrated circuit die. For example, as temperature increases, either by ambient variation or by operating heat dissipation, the plastic package tends to expand thus creating forces onto the integrated circuit die. These forces from the plastic package tend to inhibit the expansion of the die because there are expansion forces from the die itself which are opposite in direction to those of the plastic package. Also, there are expansion forces from the mounting structure which cause a frictional migratory effect on the interface between the die and the die paddle. Simply put, above 25 deg. C., expansion forces occur at two key interfaces as a result of thermal stress: 1) the integrated circuit die and plastic package interface, and 2) the integrated circuit die and mounting structure interface.
The result of these expansion forces typically appears initially as cracks on the bottom of the integrated circuit die, i.e., the surface of the integrated circuit die which interfaces with mounting structure. The cracks or fractures may then propagate throughout the die. Under lower stress conditions, the cracks may be observed on the bottom surface of the integrated circuit die as hairline fractures. These hairline fractures typically result in partial loss of function of the integrated circuit. In extreme cases, the cracks may have propagated throughout the entire thickness of the integrated circuit die to the extent of complete fissure of the die, resulting in loss of function and total failure of the integrated circuit.
Therefore, a need existed to provide a method for fabricating an integrated circuit die and attaching the integrated circuit die to a mounting structure for plastic encapsulated packages that reduces the propensity of die cracking.